Fact 1: Excluding saplings, trees weigh more than the average deer. Fact 2: All trees, including most saplings, are anchored one heck of a lot better than a deer in the road. Fact 3: The more an object is capable of resisting movement, the harder it will push back when something hits it. Fact 4: If a tree pushes back on your car, it will end badly for your car.

Conclusion: It is almost always better to hit the deer rather than the tree.